And in that same line (I don't remember all the words, so bear with me), this is one of my favorite scenes from Firefly.

Episode: Trash
Mal gets hijacked by Saffron after they successfully get the Lassiter away from Durran Haymer. After she leaves him in the desert naked, she flies to the dumpster and searches for the raygun.
Unfortunately for her, Inara, who had apparently stormed off in a huff at the beginning of the caper, had actually been a safeguard, watching for Saffron's inevitable betrayal. She had already grabbed the Lassiter, and locked Saffron in the dumpster.
The Serenity crew fly off, Mal still cheerfully naked.

Joss Whedon's first big show, Buffy the Vampire Slayer, had a ton of these, but my favorite is the season 2 episode, Innocence. There is a villain called The Judge, a demon who was ripped apart in the 14th century though not killed because "no weapon forged" could kill him; the pieces of his body were scattered, then reassembled to destroy Buffy and her friends. When she's finally had enough and is ready to confront him, he reminds her that no weapons can destroy him, and then Buffy pulls out her new toy...

It turns out the "no weapon forged" bit made sense in the 1400's, but weapons have come a long way since that.

Because missiles and rockets totally have no metal in them? It's not like she stabbed him with a wooden stick or anything.

Regardless, you asked for clever. I really like the idea of a prophecy everyone considered to be true proving false simply because it didn't account for weapons becoming a whole lot better in six centuries.